Entropy is a state function, so the standard entropy change for a chemical reaction (ΔS°_rxn) can be calculated from the difference in standard entropy between the products and the reactants.

where n_p and n_r represent the stoichiometric coefficients in the balanced equation of the products and reactants, respectively.

For example, ΔS°_rxn for the following reaction at room temperature

is computed as follows:

A partial listing of standard entropies is provided in the table.

Substance	S° (J/mol·K)
C (s, graphite)	5.740
C (s, diamond)	2.38
CO (g)	197.7
CO2 (g)	213.8
CH4 (g)	186.3
C2H4 (g)	219.5
C2H6 (g)	229.5
CH3OH (l)	126.8
C2H5OH (l)	160.7
H2 (g)	130.57
H (g)	114.6
H2O (g)	188.71
H2O (l)	69.91
HCI (g)	186.8
H2S (g)	205.7
O2 (g)	205.03

Determination of ΔS°

Consider the condensation of water, in which 1 mole of gaseous H₂O changes into 1 mole of liquid H₂O.

The standard entropy changes for the reaction, ΔS°_rxn is calculated using the standard molar entropies and stoichiometric coefficients.

The value for ΔS°_rxn is negative, as expected for this phase transition (condensation).
As a second example, consider the combustion of methanol, CH₃OH:

The same procedure is followed to calculatethe standard entropy change of the reaction:

This text is adapted from Openstax, Chemistry 2e, Chapter 16.2: The Second and Third Law of Thermodynamics.